MTNL has unlocked STD for all of its landlines. It has a dynamic locking system to lock and unlock all calls to a line .
Dynamic locking completely eliminates misuse of your telephone by others. It prevents unauthorised calls from your telephone.
This is how you use it
First remember a numeric 4 digit secret code
*ABCD -Old Secret Code
NEWC- New Secret Code

For Registration of Code DIAL 123 ABCD ABCD
and wait for a confirmation
To Change Secret Code 123 ABCD NEWC
and wait for a confirmation